I think the best
choice is to keep ibus-gtk in Recommends, as long as there are gtk2 apps in Debian.
Without ibus-gtk installed, XIM will be used as the fallback and its bugs will confuse users. There are already several open bugs about this. When ibus-gtk was in OR'ed Recommends, many users just did "apt install ibus" and had trouble with XIM unnecessarily.
Fixing the bugs is not an answer; this legacy protocol has its limits and ambiguity in the design.The ibus upstream refuses to fix XIM related bugs and I also think those bugs are not worth fixing.